DE69221329T2 - REMOVAL OF CYANIDE FROM WATER - Google Patents

Description

The present invention relates to the separation of cyanide from waste water using an adsorbent substrate treated with a water-insoluble compound.

The present invention relates to a process for separating cyanides from a cyanide-containing wastewater, in which:

(a) feeding the cyanide-containing waste water to a reaction zone containing a porous solid substrate with a water-insoluble metal compound deposited thereon;

(b) bringing the waste water into contact with the substrate which adsorbs the cyanide from the waste water; and

(c) the largely cyanide-free waste water is discharged from the reaction zone.

Another object of the present invention is a process for separating cyanide from a cyanide-containing wastewater, in which:

(a) passing the cyanide-containing waste water through a reaction zone containing a porous solid substrate with a water-insoluble metal compound deposited thereon, through which the cyanide is separated from the waste water by adsorption on the substrate;

(b) supplying a source of oxygen to the reaction zone;

(c) the adsorbed cyanide is catalytically oxidized and

(d) the treated water, which is largely free of cyanide, is discharged from the reaction zone.

Although cyanides themselves are useful industrial chemicals, certain forms of cyanide are undesirable as wastewater components because they harm the aquatic environment. Examples of undesirable forms of cyanide are HCN and CN⊃min;. As an environmental protection measure, the US Environmental Protection Agency (EPA) has set strict limits on the permissible cyanide content of industrial wastewater streams. Industrial sources of undesirable forms of cyanide include wastewater from the coal, natural gas and electroplating industries. Refinery wastewater, mainly from FCC and coking processes, must also be treated to remove cyanides.

There are already various approaches to separating cyanides. A process in which hydrogen peroxide is used to destroy cyanide in waste water has proven to be sufficient to meet the limits for cyanide discharge streams. However, this requires the continuous addition of expensive hydrogen peroxide. In addition, extensive work steps and equipment are required for the storage and handling of the hydrogen peroxide. Due to the easy decomposition of hydrogen peroxide, special precautions must be taken to ensure that no contaminants enter the storage container that could catalytically accelerate the decomposition. Since oxygen is released during decomposition, which is explosive under pressure, the container must also be closely monitored and the oxygen content kept low.

Another method of wastewater treatment to remove cyanide is chlorination, which converts the cyanide into the largely nontoxic cyanate. Chlorination processes generally use chlorine gas or hypochlorite. The disadvantage of chlorination is the cost associated with continuously adding the chlorine source to the wastewater stream, which is unfavorable compared to other processes that use chlorine. In addition, the chlorine requirement depends on the cyanide content of the water, which requires close monitoring to adjust the chlorine concentration. In addition, the process requires cooling when returning the bottoms of the HCN stripping column to the absorber to reduce the wastewater volume as effectively as possible, which increases the energy and equipment requirements of the process.

In another process for the treatment of cyanides in waste water, especially cyanides in Discharge stream from FCC and coking plants, sulfur-containing compounds such as polysulfides are injected into the cyanide-containing water. The polysulfides convert the cyanide into thiocyanate, which can be stripped from the wastewater. The disadvantage of this process, however, is that the requirements for low conversion values, as set out in the EPA specifications, are difficult to meet.

US Patent 3,650,949 describes a process for separating cyanide from waste water, in which high concentrations of copper(II) ions and oxygen are maintained in the water and the water containing cyanide, copper(II) ions and oxygen is passed into a bed of activated carbon. A water-soluble copper salt serves as the source of copper(II) ions. The copper ion must be continuously added to the process stream in order to maintain the copper content required to separate the cyanide. According to the patent, an activated carbon bed can be pre-impregnated with copper(II) ions; however, the water solubility of the copper(II) ion brings with it serious economic disadvantages. The copper is dissolved out of the bed, leaving a residue in the treated stream, which requires a further process step to separate the copper residues. In addition, the fill must be re-impregnated due to the copper dissolution. The main disadvantage of re-impregnating the fill is the downtime and the chemicals and equipment required to replace the copper.

When developing water treatment processes, special attention is given to processes that do not leave residues in the treated stream. Residues can lead to additional disposal problems. Material consumption and price also play an important role; it is therefore important to avoid processes that require the addition of expensive catalyst and reagent.

FIG. 1A shows a simplified schematic Diagram of the cyclic process according to the invention.

FIG. 1B shows a simplified schematic diagram of the continuous process according to the invention.

FIG. 2 shows a plot of ppm CN⊃min; versus bed volume, which represents the amount of CN⊃min; adsorbed on an adsorbent substrate.

FIG. 3 shows a plot of ppm CN⊃min; versus bed volume, which represents the amount of CN⊃min; adsorbed on a CuS-treated adsorbent substrate.

The invention is a process for separating cyanide from waste water, in which the waste water is passed through an adsorbent substrate treated with a water-insoluble compound. The process removes cyanides contained in the waste water to such an extent that they can no longer be detected using current analytical methods. The process is economically advantageous because it uses a long-acting adsorbent which effectively separates cyanide but does not require continuous replacement of the active adsorbent component, and the adsorbent can be easily regenerated using small amounts of inexpensive and readily available materials.

An advantage of the invention is the cost advantage and increased efficiency with which the treatment of a cyanide-containing wastewater can be carried out due to the use of a porous substrate treated with a water-insoluble metal compound which separates cyanide from the water and can be regenerated without replacing the metal. Another advantage of the invention is that the metal of the water-insoluble metal compound has oxidative properties which can facilitate the conversion of the adsorbed cyanide to harmless compounds such as cyanate and nitrogen.

The extensive removal of cyanide from wastewater, determined by current analytical methods, can be achieved by removing the cyanide-containing Water is passed over a porous substrate that has been treated with a water-insoluble metal compound that improves the adsorption and concentration of the cyanide ion on the porous substrate. An oxidizing agent is added to the substrate and the water-insoluble metal compound converts the cyanide into harmless compounds such as cyanate and nitrogen. The adsorption and oxidation take place at room temperature and normal pressure, which is easy to achieve in current refinery processes.

Porous solids are suitable as adsorbent substrates. For the purposes of the present invention, a wide variety of porous solids are possible. However, examples of porous solids include inorganic ion exchange materials and activated carbon.

Examples of inorganic ion exchange materials include naturally occurring materials such as mineral zeolites, including mordenite, clinoptilolite, erionite and sepiolite, clays, as well as synthetic materials, including Al₂O₃, SiO₂, SiO₂/Al₂O₃, zeolites such as zeolite A, zeolite X, zeolite Y as well as ZSM-5 and mordenite.

The preferred activated carbon substrate has a large surface area on which the dissolved cyanide can be physically adsorbed. Activated carbon is also a highly effective adsorbent for aqueous systems. It has an internal pore structure that can be developed by known processing methods. Activated carbons are characterized by a large specific surface area in the range of 300 to 2500 m²/g, which allows the physical adsorption of dissolved or dispersed substances from liquids or the adsorption of gases and vapors from gases. The pore size of the majority of the pores of activated carbon for gas phase adsorption is less than 3 nm, whereas the pore size of the majority of the pores of activated carbon for liquid phase adsorption is 3 nm or more. The carbon is particularly suitable for liquid phase adsorption. So The substrate may be in powdered or granular form, with particle sizes ranging from 1 to 200 mesh, preferably 4 to 40 mesh (4.76 to 0.42 mm).

A feature of the invention is that the properties of the adsorbent substrate are improved by depositing a water-insoluble compound on the surface and in the pores of the substrate, which in turn improves the service life and the cyanide adsorption efficiency of the substrate. Suitable water-insoluble compositions include, for example, those water-insoluble compounds which contain a metal with oxidative properties, examples of which are Cu, Fe, Ni, Ag, Au, Mo, Co and Zn. The water-insoluble sulfide forms of these metals are particularly preferred, although water-insoluble oxide forms are also suitable. Examples of water-insoluble sulfided compounds are CuS, Cu₂S, FeS, ZnS, MoS₂, NiS, CoS and AgS. Examples of water-insoluble oxides include Cu₂O, FeO, Fe₂O₃, ZnO, CoO, Co₂O₃, NiO and Au₂O₃. The preferred compounds have a very low solubility product constant, which makes the compounds water-insoluble for practical purposes. For example, CuS has a solubility product constant of 8.5 x 10⁻⁴⁴ (at 18ºC), expressed as the solubility of CuS in water, i.e. the amount of CuS that dissolves in one liter of water. CuS is practically insoluble in water with a very low solubility of approximately 5.9 x 10⁻²¹ g Cu/l.

A packed bed provides an effective and efficient contactor. In the packed bed, the reaction zone advances in the direction of flow as the adsorbent becomes increasingly exhausted. The exhaustion of the adsorbent can be detected by the breakthrough of cyanide. The packed bed can be easily regenerated by passing hot gases, such as steam or flue gas, over it. In the regeneration gas, a Oxidizing agents are included. Air, which is easily accessible, is preferred as an oxidizing agent, but steam, ozone, O₂ and polysulfides can also be considered. The mechanism of this process can be described as follows:

2 CN&supmin; + O&sub2; T 2 CNO&supmin;

2 CNO&supmin; + O&sub2; T N&sub2; (g) + 2 CO2 (G)

CNO⊃min; and N₂ are known to be non-hazardous to the environment and can be easily disposed of.

The invention is practiced in the liquid phase in a cyclic process consisting of two steps, adsorption and regeneration. In a preferred embodiment of the cyclic process, shown in FIG. 1A, in the adsorption step, waste water is fed via line 13 to a reaction zone 11 containing the CuS-treated adsorbent. The waste water flows at a liquid hourly space velocity (LHSV) of from 0.01 to 100, preferably from 0.1 to 10. The flow rate of the water is determined by the pressure exerted on the stream by an upstream treatment unit. The reaction temperature ranges from 4 to 204°C (40°F to 400°F) and reaction pressures range from 0 to 1000 psi gauge (1-70 bar). The cyanide is adsorbed on the adsorbent and the treated water flows out of the first reaction zone via discharge line 17. Cyanide breakthrough, ie the detection of cyanide in the treated water exiting discharge line 17, indicates that the adsorbent is exhausted. Preferably, the water flow is interrupted shortly before breakthrough, which occurs after treatment of about 100 to 100,000 bed volumes of water, depending on the cyanide content of the water, and the process is switched to the regeneration step by supplying the oxidizing agent to the adsorbent via line 19. The oxidizing agent, optionally in a mixture with clean water, is conveyed through line 21. For regeneration, the oxidizing agent and optionally water are passed through a Load of 1 to 1000 LHSV and 0 to 100 psi overpressure (1-7.9 bar) in countercurrent through the reaction zone 11. The oxidized cyanide ion and other oxidizable components of the water are carried out of the reaction zone by wastewater during the next pass or are washed off the adsorbent and disposed of properly.

The process can also be carried out in a continuous contactor in the liquid phase, with adsorption and regeneration taking place in one step. In the continuous process, according to FIG. 1B, the adsorbent bed used in reaction zone 35 corresponds essentially to the adsorbent bed used in reaction zone 11 in FIG. 1A. However, by adding the oxidizing agent during adsorption, the bed is regenerated so that the cyanide-containing water flow does not have to be interrupted to regenerate the adsorbent bed. The waste water 31 fed in through line 31 and the oxidizing agent, preferably air, fed in through line 33 are conveyed to the fixed bed 35 with a load of 0.01 to 100 LHSV, preferably 0.1 to 10 LHSV. The amount of oxidant supplied with the water is sufficient to provide 1 to 1000 times, preferably 1 to 100 times, the amount stoichiometrically required to oxidize the adsorbed oxidizable components, including cyanide (i.e., the chemical oxygen demand of the water). The reactor 35 is maintained at wastewater stream temperatures ranging from 2 to 93°C (35 to 200°F), preferably 4 to 66°C (40 to 150°F), and pressures ranging from 0 to 1000 psi (0-69 bar), preferably 0 to 500 psi (0-34.5 bar). The cyanide contained in the water is adsorbed on the bed, whereby the adsorbed cyanide is simultaneously oxidized by the oxygen flowing over the bed. The treated discharge stream is then discharged via line 37 into a separating device 39, in which the air is separated and discharged via opening 43 and the treated water is discharged via line 45.

The effective water treatment properties of the bed are quite long-lasting, since the metal deposited on the substrate, which is present in the form of a water-insoluble compound, does not need to be replaced. However, as the bed ages, the adsorption properties decrease and the bed must be regenerated. The regeneration process described above using hot gas containing an oxidizing agent can be used for this. In addition, a process for reactivating the adsorbent has also been discovered, which is an additional feature of the invention. The service life of the adsorbent can be extended by passing ammonia over the bed. Reactivation of the bed by adding ammonia should only be necessary when the first signs of cyanide breakthrough appear or, depending on the cyanide content of the water, after treatment of 100 to 10,000 bed volumes. By adding ammonia, the adsorbent is reactivated and its service life is extended without the need to renew the metal compound. The ammonia can be added in parallel with the untreated waste water and the oxidizing agent. Alternatively, the ammonia can also be passed through the bed in countercurrent, as in the cyclic process. Effective regeneration of the adsorbent is achieved with a concentration of 1 to 100 ppm, preferably 2 to 10 ppm, of ammonia. The ammonia can be added without changing the operating conditions.

According to one embodiment of the invention, water-insoluble CuS is deposited on the surface and in the pores of the activated carbon substrate. To prepare this adsorbent, an activated carbon substrate is impregnated with an aqueous copper nitrate solution. The substrate is then dried at 38 to 371ºC (100ºF to 700ºF), preferably at 149 to 260ºC (300ºF to 500ºF), whereby the dried copper nitrate decomposes to copper (II) oxide (CuO). To obtain an active adsorbent, the temperature should not exceed 371ºC (700ºF). The amount of CuO on the adsorbent is preferably from 1 to 30%, preferably from 5 to 20%, based on the weight of the adsorbent treated. The CuO deposited on the adsorbent is then sulfided by contacting the bed at 38 to 260°C (100 to 500°F) with a source of sulfur, preferably hydrogen sulfide or elemental or organic sulfur, dissolved in a solvent, preferably oil. The adsorbent bed is then dried at elevated temperatures of 38 to 260°C (100 to 500°F), preferably 93 to 149°C (200 to 300°F). Sulfiding can be done externally, ie, ex-situ. Sulfiding should be continued until the sulfur content is at least 80% of the stoichiometric amount of copper(II) oxide required to form the metal sulfide, ie CuS. In external sulfidation, the finished adsorbent is introduced into the reactor to produce the bed.

The wastewater should be maintained at a pH of about 6-12; however, ideally the water is neutral to slightly alkaline with a pH of 7 to 10.

It is expected that the process will not only be useful in separating cyanides, but will also remove other aggressive and undesirable contaminants such as mercury from the wastewater. The process will also oxidize other reducing compounds, thereby reducing the chemical oxygen demand (COD) of the wastewater and improving the quality of the wastewater for discharge.

The effluent stream from the FCC and coking plants represents a problematic source of cyanide in wastewater for refineries. An example of the composition of wastewater from an FCC plant is shown in Table 1. Table 1 Composition of FCC wastewater

When treating wastewater from FCC or coker plants, the process of the invention is best arranged prior to mixing the wastewater streams from the FCC and coker plants with other refinery wastewaters that would reduce the cyanide concentration and increase the volume of water to be treated.

The advantages of the invention will now be explained in more detail using the following examples.

example 1

To prepare test solutions that are intended to simulate cyanide-containing wastewater from an FCC plant, NaCN is dissolved in demineralized water to give a cyanide content of 10 or 100 ppm. To obtain the cyanide, the pH of the solution is raised to 10 by adding NaOH.

Adsorbents 1 and 2 are prepared, which have the properties listed in Table 2. Table 2 Adsorbent properties

Adsorbent 1 is prepared by impregnating the activated carbon with a Cu(NO3)2 solution to the required copper content. The mixture is dried at 104ºC (220ºC) for 3 hours. The dried product is then heated to 260ºC (500ºF) in air to decompose the Cu(NO3)2 to CuO. The treated adsorbent is then sulfided by pumping 0.1% sulfur naphtha through the bed at 232ºC (450ºF) and 250 psi (17.2 bar). Sulfiding is complete when H2S can be detected in the effluent stream.

Adsorbent 2 is a conventional activated carbon adsorbent sold by Calgon under the trade name CAL 12X40.

To prepare the bed, 5 cm³ of the adsorbent, which has been crushed to a size of 20 to 40 mesh (0.841-0.42 mm), are filled into a glass tube with a diameter of 7 mm.

The CN⊃min; concentration of the water is measured using a cyanide ion-specific electrode. The sensitivity of the electrode is 0.01 ppm CN⊃min;.

Example 2

A water test solution containing 10 ppm cyanide is passed downward through the bed of activated carbon adsorbent at a rate of 5 LHSV, i.e. 25 cm3 per hour. The reaction conditions are a temperature of 23ºC and a pressure of 100 psi (6.9 bar). After each bed volume treated, the effluent is collected and analyzed for its cyanide content. The cyanide breaks through after about 7 bed volumes. The values obtained are presented in a plot of bed volume against ppm cyanide (see FIG. 2).

Example 3

A water test solution containing 10 ppm CN⊃min; is passed downwards through the packed bed of adsorbent 1 according to Example 1, namely the activated carbon treated with CuS. The reaction conditions correspond to those selected in Example 2. After certain volumes of water treatment, the cyanide content of the treated discharge stream is determined. At a cyanide concentration of 10 ppm, the cyanide is still very well removed from the water by the activated carbon adsorbent treated with CuS even after 80 bed volumes. The values obtained are shown in a plot of the bed volume against ppm cyanide (see FIG. 2).

Example 4

In a more stringent test, a water test solution containing 100 ppm cyanide is passed over the CuS-treated activated carbon adsorbent. The reaction conditions are a temperature of 23ºC and a pressure of 100 psi (6.9 bar). The cyanide breaks through after about 35 bulk volumes.

Example 5

In the test described in Example 4, after 35 bed volumes, air is fed in countercurrent to the test solution at a rate of 240 cm³ air/cm³ H₂O to oxidize the adsorbed cyanide. After passing bed volumes of water and air through the bed, the liquid product is analyzed for its CN⊃min; concentration. No cyanide can be detected. After further passage of air and water through the adsorbent, no cyanide can be detected in the treated water even after 135 bed volumes. This indicates that the adsorbed cyanide is easily oxidized with air.

A comparison of the test results shown in Figures 2 and 3 shows that a CuS-treated activated carbon carrier is far superior to the activated carbon itself in adsorbing cyanide from wastewater. At a cyanide concentration of 10 ppm, the cyanide breaks through the activated carbon after only 7 bed volumes. In contrast, at the same cyanide concentration most commonly found in refinery effluents, by treating the wastewater over a bed of activated carbon treated with CuS, cyanide is still effectively removed from the wastewater even after 80 bed volumes. Due to the range of variation in the test conditions in FIG. 2, about 0.75 ppm of cyanide appears in the product after 30 bed volumes; however, this value immediately drops to 0 after 40 bed volumes and should not affect the performance of the process.

According to FIG. 3, at a cyanide concentration of 100 ppm, the CuS-treated substrate effectively removes the cyanide from the water until the cyanide breaks through after about 35 bed volumes. The air is supplied in parallel with the water. After regeneration with air, the discharge stream is again cyanide-free after 40 bed volumes. The discharge stream remains cyanide-free even after 135 bed volumes, which is not shown in FIG. 3. This result suggests that the process can be carried out over longer periods of time without having to carry out regeneration by supplying air together with the waste water. This result also shows that the adsorbent can be regenerated on-line without having to shut down the plant to supply air.

Example 6

To assess the long-term use of the CuS-treated adsorbent, the water test solution containing 10 ppm cyanide is passed downward through the CuS-treated activated carbon bed. The solution is treated above the bed with 1 LHSV. The temperature is maintained at 110ºC, the pressure at 200 psi (13.8 bar) and the oxygen flow at 20 cc/min. The adsorbed volume is 3.5 cc. The values obtained are shown in Table 3. After 582 bed volumes, the product contains 0.1 ppm cyanide, indicating cyanide breakthrough. Increasing the LHSV to 10 results in no further cyanide breakthrough, indicating that cyanide adsorption and reaction are rapid. After reducing the LHSV to 1, the Cyanide passes through after 731 bed volumes at 10 ppm (100%); ie the bed is completely exhausted. To reactivate the exhausted bed in order to extend the life of the bed, 100 ppm of ammonia is added to the feedstock after 740 bed volumes, and after 761 bed volumes no more cyanide can be detected in the discharge stream, which suggests that the exhausted adsorbent is reactivated by adding a small amount of ammonia (100 ppm) to the feedstock. Table 3 Regeneration of the water-insoluble adsorbent with NH3

Claims (20)

1. Process for the separation of cyanides from a cyanide-containing waste water, comprising: (a) feeding the cyanide-containing waste water to a reaction zone containing a porous solid substrate with a water-insoluble metal compound deposited thereon; (b) bringing the waste water into contact with the substrate which adsorbs the cyanide from the waste water; and (c) the largely cyanide-free waste water is discharged from the reaction zone. 2. Process according to claim 1, in which a water-insoluble metal compound is used which contains Cu, Fe, Ni, Ag, Au, Mo, Co or Zn. 3. Process according to claim 2, in which a sulfide is used as the water-insoluble metal compound. 4. Process according to claim 2, in which activated carbon, Al₂O₃, SiO₂, SiO₂/Al₂O₃ or a zeolite is used as the porous solid substrate. 5. Process according to claim 3, in which CuS, Cu₂S, AgS, ZnS, M0S₂, CoS, NiS or FeS is used as the water-insoluble metal compound. 6. Process according to claim 2, in which the water-insoluble metal compound used is Cu₂O, Au₂O₃, ZnO, Fe₂O₃, CuO, Co₂O₃, NiO or FeO. 7. Process according to claim 1, in which the cyanide-containing wastewater is fed to the reaction zone with a load of 0.01 to 100 LHSV. 8. Process for the separation of cyanide from a cyanide-containing waste water, which comprises: (a) passing the cyanide-containing waste water through a reaction zone containing a porous solid substrate with a water-insoluble metal compound deposited thereon, through which the cyanide is separated from the waste water by adsorption on the substrate; (b) supplying a source of oxygen to the reaction zone; (c) the adsorbed cyanide is catalytically oxidized and (d) the treated water, which is largely free of cyanide, is discharged from the reaction zone. 9. Process according to claim 8, in which activated carbon is used as the porous solid substrate. 10. The method according to claim 9, wherein a water-insoluble metal compound is used which contains Cu, Fe, Ni, Ag, Au, Mo, Co or Zn. 11. Process according to claim 10, in which a sulfide is used as the water-insoluble metal compound. 12. The method of claim 8, further comprising reducing the chemical oxygen demand of the water by oxidizing the oxidizable components of the water. 13. The method of claim 11, wherein the water-insoluble metal sulfide compound is deposited on the porous solid substrate by impregnating the substrate with an aqueous copper nitrate solution, drying the substrate, whereby the copper nitrate decomposes to copper (II) oxide, and sulfiding the copper (II) oxide on the substrate by contacting the substrate with a sulfur source until the sulfur content is at least 80% of the stoichiometric amount of copper (II) oxide. 14. A process according to claim 8, wherein air is used as the oxygen source. 15. A process according to claim 8, wherein the substrate is reactivated by feeding ammonia into the reaction zone. 16. The method according to claim 1, wherein activated carbon is used as the porous solid substrate and a metal sulfide or oxide is used as the water-insoluble metal compound. 17. Process according to claim 16, in which CuS is used as the metal sulfide. 18. The process of claim 16, wherein an oxygen source is supplied to the reaction zone for oxidizing the adsorbed cyanide. 19. A process according to claim 18, wherein the treated substrate is treated by feeding ammonia into the Reaction zone reactivated. 20. The process of claim 16, wherein a porous activated carbon substrate is used which contains 1 to 30 wt.% water-insoluble sulfided copper compound.